This invention relates to a housing for mounting a microphone and more particularly to apparatus for mounting a unidirectional microphone in a hands-free telephone subset.
Present hands-free telephone subsets generally utilize an omnidirectional electret condenser microphone which is mounted directly inside the subset. As one can ascertain, there are many examples of hands-free telephone subsets which essentially enable one to engage in a conversation without accessing the typical handset associated with the telephone subset. The handset is a conventional device and is coupled to the base of the subset by means of a suitable cable. The handset contains a receiving portion and a transmitting portion. In any event, in the hands-free telephone the receive line is coupled to a loudspeaker whereby the calling party's voice is amplified by means of the loudspeaker. The called party, engaging in the conversation, hears the calling party via the loudspeaker and can communicate with the calling party via a microphone which is built into the subset. In this manner, one can achieve hands-free operation while engaging in a telephone conversation.
As indicated above, many available hands-free telephone subsets utilize an omnidirectional electret condenser microphone which is mounted directly inside the subset. No special mounting techniques are employed. The microphone usually is installed in a neoprene "rubber boot" which attaches directly to the front face of the subset. Both the frequency response and sensitivity are fairly good and the cost is relatively inexpensive. The electret condenser microphone is a well-known component and available from many sources. The electret microphone utilizes a foil electret and requires no polarizing voltage. The frequency response of such devices is within plus or minus 3 db from 50 to 15,000 Hz or greater. Sensitivities of 50 microvolts have been achieved and greater sensitivities are available.
Essentially, the device has a relatively low impedance and can be connected directly to a conventional transistor amplifier. The device is a capacitive-type microphone where the electret capacitor microphone depends on its operation on minute vibrations of capacitance which are produced as sound vibrations impinge on one flexible plate of the capacitor head. In any event, in the case of the electret microphone, the flexible plate is a foil electret constructed of a thinly metalized sheet of fluorocarbon or polycarbonate. The foil contains a permanent static charge. Since its spacing is varied from the fixed element, the electrostatic field is varied thus producing a varying voltage at the output terminals. In any event, as indicated above, such devices are available from many manufacturers for both omnidirectional and unidirectional operation. See, for example, the devices manufactured by Cord Electronics of Westbury, N.Y. There are many other manufacturers who manufacture such devices as United Chem-Con of Rosemont, Ill. and so on.
As indicated, the microphone is available in a unidirectional or omnidirectional mode. Essentially, one can obtain a unidirectional pattern in such a device whereby the response pattern is sometimes referred to as a cardioid pattern. Such patterns are well-known and there are many different types of microphones which will exhibit such patterns. See, for example, a text entitled "Electronic Engineer's Handbook" second edition, published 1982, Chapter 19, pages 19-39 to 19-50. This text shows many examples of existing microphones and their particular responses. In any event, as indicated above, based on prior art operation, most subsets utilize an omnidirectional electret microphone. Since the microphone has an omnidirectional response pattern, it is equally sensitive to sounds arriving at the front, rear or sides of the microphone and essentially cannot distinguish between wanted and unwanted sounds. As a result, even though the subset's loudspeaker is usually placed behind the microphone, unwanted pickup of the loudspeaker signals can and do occur. The microphone's output consists of a mixture of signals both from the loudspeaker and from the human speaker's voice. This composite signal confuses the voice switching control circuits of the subset and can cause false switching, clicking and lockout to occur. In any event, a unidirectional microphone, as opposed to an omnidirectional microphone, would help overcome these defects and further reduce unwanted noise from indirect sources. Hence, it would be desirable to utilize a unidirectional microphone as a microphone on a hands-free telephone set to avoid the above-noted problems.
It is therefore an object of the present invention to provide a housing to accommodate a unidirectional microphone and to assure that the interference from the loudspeaker associated with a telephone subset and other indirect sources will not interfere with the subset's operation in a hands-free mode.